Wireless communication systems have been evolving substantially over the last two decades. The explosive growth of the wireless communication market is expected to continue in the future, as the demand for all types of wireless services is increasing. New generations of wireless mobile radio systems aim at providing higher data rates and a wide variety of applications to the mobile users, while serving as many users as possible. However, this goal must be achieved under the constraint of the limited available resources, including spectrum and power. Given the high price of the spectrum and its scarcity, the systems must provide higher capacity and performance through a better use of the available resources. The increasing spectrum shortage gives rise to a necessity for immediate solutions regarding the spectrum usage.
A strong future solution is the cognitive radio which aims at very efficient spectrum utilization employing smart wireless devices with sensing, learning, and adaptation capabilities. One of the main issues regarding the success of the cognitive radio is the development of high performance radio access technologies that can accommodate the requirements given above with highly adaptable transmission formats.
Ultrawideband (UWB) is becoming an attractive radio access solution for wireless communications, particularly for short and medium range applications. According to the modern definition, any wireless communication technology that has a bandwidth wider than 500 MHz or a fractional bandwidth greater than 0.2 can be considered a UWB system.
A basic technique considered for implementing UWB is the impulse radio, which is based on transmitting extremely short (on the order of nanosecond) and low power pulses that have a very wide spectrum. In FIG. 1, a time hopping ultrawideband (TH-UWB) system is demonstrated. Each information carrying symbol is transmitted with a number of pulses, where in this case four pulses represent a symbol. Pulses occupy a location in the time-frame based on the specific pseudo random (PN) code assigned for each user. Two different codes and the corresponding pulse locations are shown with reference to FIG. 1. Note that these two codes are orthogonal, and as such they do not interfere with each other. The pulses of another user that interferes with the code of the first user are also shown to demonstrate how interference from other users affects the system. Each block in this figure represents a number of symbols, where forward error correction (FEC) coding, interleaving, and other MAC layer protocols might be applied. Impulse radio is advantageous in that it eliminates the need for up and down-conversion, and allows transceivers of low-complexity. It also enables the employment of various types of modulations, including on-off keying (OOK), pulse amplitude modulation (PAM), pulse position modulation (PPM), and binary phase shift keying (BPSK), as well as the use of different receiver types such as energy detectors, RAKE, and transmitted reference receivers.
Another strong candidate for UWB communication is the multi-carrier approach, which can be realized using Orthogonal Frequency Division Multiplexing (OFDM) as illustrated with reference to FIG. 2. OFDM has become a very popular technology due to its special features such as robustness against multipath interference, ability to allow frequency diversity with the use of efficient FEC coding, capability of capturing the multipath energy, and ability to provide high bandwidth efficiency. OFDM can overcome many problems that arise with high bit rate communications, the most significant of which is the time dispersion. In OFDM the data bearing symbol stream is split into several lower rate streams, and these streams are transmitted on different carriers. Since this increases the symbol period by the number of non-overlapping carriers, multipath echoes affect only a small portion of the neighboring symbols. Any remaining ISI can be removed by cyclically extending the OFDM symbol.
In terms of adapting the transmission parameters, OFDM offers many possibilities, including the ability to adapt the transmit power, the cyclic prefix size, the modulation and coding, and the number of sub-carriers. In addition to adaptation over each packet, in the case of single carrier system, OFDM also offers adaptation of the parameters for each carrier or over a small group of carriers. In other words, adaptation can be done independently over narrower bands rather than the entire transmission band. Similarly, reception of an OFDM signal offers new designs and approaches for adaptive receivers.
In a UWB system, the unlicensed usage of a very wide spectrum that overlaps with the spectra of narrowband technologies brings about some concerns. Therefore, a significant amount of research has been carried out to quantify the effect of UWB signals on narrowband systems. The transmitted power of UWB devices is controlled by the regulatory agencies (such as the FCC in the United States), so that narrowband systems are affected by UWB signals only at a negligible level. Therefore, UWB systems are allowed to co-exist with other technologies under stringent power constraints. This fact puts significant limitation on the variety of applications, maximum data rate, and transceiver design options, and the UWB system becomes very susceptible to the effects of the narrowband systems. Systems with a spectral allocation similar to UWB are known in the art as underlay systems, or shared unlicensed systems. The severe power limitations on underlay systems restrict their usage to only very short range devices. Therefore, all current UWB efforts are in the direction of making UWB systems work in an underlay scenario with a focus only on wireless personal area networks (WPAN).
In communications system design, dealing with interference is one of the main considerations. Interference can be defined as any kind of signal received aside from the desired signal and noise. According to its origin, interference can occur in two ways: (1) Self-interference, which is caused by the transmitted signal due to improper system design. Examples of self-interference include inter-symbol (ISI), inter-carrier (ICI), inter-frame (IFI), inter-pulse (IPI), and cross-modulation (CMI) interferences. Self-interference can be handled by properly designing the system and transceivers. (2) Interference from other users, which can be further categorized as: (a) Multi-user interference, which is the interference from users using the same system or a similar technology. Co-channel and adjacent channel interferences belong to this category. Multi-user interference can be overcome by proper multi-access design and/or employing multi-user detection techniques. (b) Interference from other types of technologies. This kind of interference mostly requires interference avoidance or cancellation. It is more difficult to handle compared to multi-user interference, and can often not be suppressed completely. Narrowband interference (NBI) is a well-known example of this type of interference.
UWB systems operate over extremely wide frequency bands, where various narrowband technologies also exist with much higher power levels as illustrated with reference to FIG. 3. The influence of these narrowband technologies on the UWB system can be significant, and in the extreme case, these signals may jam the UWB receiver completely. Even though narrowband signals interfere with only a small fraction of the UWB spectrum, due to their relatively high power with respect to the UWB signal, the performance and capacity of UWB systems can be affected considerably. The recent studies show that the bit-error-rate (BER) performance of the UWB receivers is greatly degraded due to the impact of narrowband interference. The high processing gain of the UWB signal can cope with the narrowband interferers to some extent. However, in many cases, even the large processing gain alone is not sufficient to suppress the effect of the high power interferers. Therefore, either the UWB system design needs to consider avoiding the transmission of the UWB signal over the frequencies of strong narrowband interferers, or the UWB receivers must employ NBI suppression techniques to improve the performance, the capacity, and the range of the UWB communications.
NBI is not a recent problem. For other wideband systems such as the code division multiple accessing (CDMA) system, this issue has been studied extensively. In these systems, NBI is partially handled with the processing gain, and by employing interference cancellation techniques including notch filtering, predictive techniques, minimum mean square error (MMSE) detectors, and transform domain techniques. However, the NBI problem in UWB is more challenging due for a variety of reasons. First, compared to the licensed CDMA systems, the unlicensed UWB extends a much wider frequency band, but transmits less power, thus forcing the UWB system to coexist with a higher number of powerful interferers. Second, in carrier modulated wideband systems, the received signal is down-converted to the baseband and sampled above the Nyquist rate, which allows it to be processed digitally. However, the UWB signal, being already in the baseband, can not be sampled at the Nyquist rate with the existing technology. Therefore, the numerous NBI suppression techniques proposed for other wideband systems, which can be realized by means of advanced signal processing methods, are not applicable to UWB systems.
In the literature, there are numerous influential studies focusing on NBI suppression for UWB systems. The methods proposed in these studies can be classified as avoidance and cancellation techniques. NBI avoidance methods are based on avoiding transmission over the frequencies of strong narrowband interferers. Multi-carrier approach, multi-band schemes, and pulse shaping are among the various avoidance methods. The cancellation methods, on the other hand, aim at eliminating the effect of NBI on the received UWB signal. MMSE combining, frequency domain techniques such as notch filtering, time-frequency methods like wavelet transform, and time domain approaches constitute the primary cancellation methods.
Accordingly, what is needed in the art is a flexible and adaptable radio access technology that can take advantage of the available spectrum in an opportunistic manner.